1. Field of the Invention
The invention relates to a light emitting device, and more particularly to a light emitting diode covered with a reflective layer.
2. The Prior Arts
A light emitting diode (LED) is a semiconductor optoelectronic device that can generate light. FIG. 1 illustrates a typical light emitting diode. The light emitting diode 10′ includes a substrate 11′, a first semiconductor layer 12′ (n-type semiconductor layer) formed on the surface of the substrate 11′, and a active layer 13′ and a second semiconductor layer 14′ (p-type semiconductor layer) formed on the first semiconductor layer 12′ subsequently. The light emitting diode emits enough photons by giving off energy in the form of light during the process of recombining the electrons with the holes near the p-n junction area in order to achieve the desired illuminating effect. Light emitting diodes have the features of small size, light weight, high illuminating efficiency, and low manufacturing cost so that they are typically used as the light sources of display devices, traffic signals and optical fiber communications nowadays. However, the technical bottleneck for the light extraction efficiency of light emitting diodes still need to be made breakthrough because the light emitted by recombining the carriers is radially scattered so that the light is not all emitted from the direction of light extraction. As a result, the brightness of light emitting diodes cannot reach the desired value, and also too much heat is generated during light scattering.
To overcome the drawback of low light extraction efficiency, a few literatures disclosed methods for improving the light extraction efficiency of LED. However, all those methods have drawbacks and limits thereof. For example, in U.S. Pat. No. 6,155,699, Miller et al. disclosed a distributed Bragg reflector (DBR) structure comprised of multiple alternating layers of high and low refractive index materials as a reflective layer of LED to enhance the light extraction efficiency of LED.
In another example, Taiwan patent no. 541,728 disclosed a highly reflective dielectric stack formed on the mesa wall of a flip-chip LED. The dielectric stack is composed of alternating low refractive index and high refractive index layers. The highly reflective stack reduces the light lost through the mesa wall by reflecting and thereby redirecting a larger portion of the guided light within the LED chip which is incident on the coated mesa wall. However, the highly reflective dielectric stack was only formed on the mesa wall of a flip-chip LED, and the light lost still occurred on the other side surfaces. Furthermore, the highly reflective dielectric stack can be formed only on the mesa wall having a certain range of tilt angles. Moreover, the stack is fabricated by a general photolithographic process, and thereby the manufacturing cost is extremely high.
In another example, Taiwan patent no. 488,089 disclosed a light emitting diode having an optical reflective film which was used for reflecting the light transmitted through the transparent substrate to enhance the illuminant effect. However, the light transmitted through the side surfaces of the light emitting diode can not be reflected by the optical reflective film so that the improvement for the light extraction efficiency of LEDs is limited. Taiwan patent no. 577,178 disclosed a high efficient light emitting diode having reflective metal layer, and a structure which can prevent the reflective metal layer from reacting with the p-type ohmic contact during an annealing process. However, the light transmitted through the side surfaces of the light emitting diode can not be reflected by the optical reflective film so that the improvement for the light extraction efficiency of LEDs is limited.
From the reflective structures of the conventional LEDs, it can be seen that only portions of light or a certain wavelength of light can be reflected by DBR or optical reflective film structures. Meanwhile, these structures are fabricated by the complicated photolithographic and etching process. To overcome the drawbacks of conventional reflective structures, a method for improving the light extraction efficiency and simplifying the fabrication of the reflective structure need to be provided in order to meet the requirements of the rapid advancements of products.